I find that a deck of cards is useful in adjusting the vta. On my particular tonearm I have a cueing lever collar at base of tonearm. There is a gap there between the plinth and the collar. I take a base measurement of that space with numerous stacked cards and count the total amount of cards it took to "take up" that space. That’s my arbitrary starting point or reference, as that space will differ depending on where that collar was set at factory from table to table (in my case pro-ject). I then add or subtract cards to raise or lower the arm, upon loosening vta adjustment screws. I log the amount of cards used for each particular cartridge and use as reference. Been doing this for years with great success. In the end, it is your ears that are the final judge on the "sweet spot". However, That sweet spot won’t be optimal for all records. That is virtually impossible to achieve due to varying thickness of vinyl and quality of the pressing etc. You can only obtain a happy medium/average, unless you want to futz with it every time you play a record.
Cart adjustment, that moment when you nail it!
Dang, after futzing with my analog system, and getting it “ok” but with some distortion here and there etc, FINALLY got the thing dialed. All I can say is WOW... Lot’s of trial and error, tiny adjustments, etc. and then ding! Nailed it. Finally! Just awesome. If you are in the tinker mode, keep trying, you WILL get there. It’s worth the time!